Fix CollectionView not displaying header or footers

[jknaudt21]

Problem

The footer and header's of a CollectionView would not display on Windows.

This was happening because they were created as WrapperControl in ViewToHandlerConverter.cs , and when being initialized they didn't have parents (although they should!). This, in turn, prevents them from getting the appropriate MauiContext when calling view.FindMauiContext(). Without the context, they can't render.

The reason why they ended up having no parents was because we make a call to ClearLogicalChildren when updating an ItemsSource. This was made to prevent memory leaks but had the unintended side effect of not rendering the headers and footers. #13530

Solution

We ensure to only clear up the items in the ItemSource of the CollectionView. That way any logical child that should persist is not wiped, and can thereby render correctly.

Before & After

I'm using an empty collection view with a footer and header

Sample code

    <VerticalStackLayout>
        <CollectionView MaximumWidthRequest="300">
            <CollectionView.Header>
                <Button BackgroundColor="AliceBlue" Text="Foo"/>
            </CollectionView.Header>
            <CollectionView.Footer>
                <Button Text="Bar" TextColor="Red" />
            </CollectionView.Footer>
        </CollectionView>
    </VerticalStackLayout>

Before	After
(Nothing rendered)

Issues Fixed

Fixes #14557

Remaining work

• Write a test

[samhouts]

Paging @jonathanpeppers for memory leaks implications

[jknaudt21]

Got spooked on memory leaks so I bolstered the current memory leak test.

[rmarinho]

Failing tests on android

[jknaudt21]

Failing tests on android

So this seems to be a new bug. The Android collection view leaks if it has a header or footer :(

Edit: The failing Android tests and leak have been fixed: #17012

Requested changes have been made